Young wirtwood trees are actually sapient, known once to the ancients as the "elders," repositories of knowledge about magic and the natural world. Modern peoples have lost this knowledge, however, due to the nature of the wirtwood's seeding. Mature trees, which takes upwards of a millennium to fully grow, do not reproduce through the normal methods of seeded fruit as is so common in other trees. Instead, the maturation and seeding of a wirtwood is a nearly apocalyptic event for those who live in the immediate region.
A wirtwood seeds only once in its life, during which it dies. The magic power bound up into the tree is released, destroying the tree in similar manner to burning, but releasing far more smoke than it should. What look like great, massive thunderclouds -- all perfectly black -- gather and sweep across the sky. Where the wirtwood's magical smoke blots out the sky, a black fog spreads across the land beneath, the seeds of the wirtwood saturating the earth.
Living creatures caught in a wirtwood's seeding fog are as good as dead from the first breath they take. The minds of people and animals alike are destroyed by breathing the fog, leaving insensate bodies that might as well be corpses but for the fact that they still breathe, their hearts still beat. In the worst cases, whole spans of a hundred miles around can be wiped out by a large wirtwood going to seed. It was some of these revered wirtwood "elders" which grew to maturation and annihilated the greatest cities and centers of power of the ancients, who had transplanted great wirtwoods around the known world so they might honor the trees and learn from them. The survivors of the ancients, when they learned what happened, destroyed all the younger wirtwoods they could find to prevent this from happening again, including their own village trees. In this manner did ancient society collapse and knowledge of the wirtwoods was lost. The trees themselves, however, did not go extinct. Each wirtwood has maybe four or five viable seeds in the storm of billions that it throws out, and so the trees have grown up again in the past millennium, with none knowing what disaster they harbor.
Rarely, so rare as to be as yet unknown, a single person may survive the seeding of a wirtwood tree. They do not come out of the experience unchanged -- whether through the influence of the seed-smoke or simply living through the horrors of such an event -- but they retain a functioning mind. Such a person is marked specially by the wirtwoods, though to what end nobody knows. Such purposes could be fair or foul, as nobody even knows whether the wirtwoods maliciously kept the secret of their seeding or are simply victims of their own need to reproduce and the horrible means by which it is done.